The addition of HBr has been used with oxygen and chlorine-based chemistries to improve selectivity of polysilicon to gate oxide in gate-stack etching. As a consequence of high selectivity, polymer residues become a major factor in critical-dimension (CD) control. It is believed that the presence of HBr in the plasma is responsible for polymer formation. HBr and its polymer residues may induce surface reactions to form thin oxide layers. Such a phenomenon has been observed if the wafers are not treated with HF vapor immediately after reactive ion etching (RIE) of the gate. The magnitude of the oxide film growth is proportional to the delayed time between RIE and HF vapor treatment. The sidewall thickness of the gate is also affected by the time-delay effect. The growth of the oxide film on the sidewalls can eventually affect the gate CD, and thus the device performance. A simple reaction model for the growth of the oxide film is proposed to explain the correlation between delayed time and CD bias.